add python-is-python3 and portage to bootstrap container

[trz42]

The task to install eselect-repository failed without python-is-python3 and portage.

For the former, see https://unix.stackexchange.com/questions/609855/how-to-make-python-an-alias-of-python3-systemwide-on-debian/617643#617643

For the latter, just the portage python module was missing.

With this PR both packages are added to the container. The building of the compatibility layer finishes.

Not entirely sure if these packages shall be provided by the container or the bootstrapped prefix.

[eessi-bot[bot]]

Instance eessi-bot-citc-aws is configured to build:

• arch x86_64/generic for repo EESSI-pilot
• arch x86_64/intel/haswell for repo EESSI-pilot
• arch x86_64/intel/skylake_avx512 for repo EESSI-pilot
• arch x86_64/amd/zen2 for repo EESSI-pilot
• arch x86_64/amd/zen3 for repo EESSI-pilot
• arch aarch64/generic for repo EESSI-pilot
• arch aarch64/graviton2 for repo EESSI-pilot
• arch aarch64/graviton3 for repo EESSI-pilot

[amadio]

Cannot comment on whether python-is-python3 is needed, but portage most definitely should come from Gentoo prefix, and should always be there.

[trz42]

Made it a draft, because it might not be needed after all.

[trz42]

Adding

  vars:
     ansible_python_interpreter: "{{ gentoo_prefix_path }}/usr/bin/python3"

to tasks that use community.general.portage makes sure the python3 from the prefix is used.

Hence, this PR is not needed anymore.